Less attractive to lygus, while aphids prefer it

With irrigation water supplies growing increasingly more scarce, San Joaquin Valley cotton producers know they will face the issues of deficit irrigation more often in the future.

It doesn't take a rocket scientist to realize that when cotton receives less than optimal irrigation, it will yield less. University of California Cooperative Extension farm advisor for Fresno County Dan Munk said that irrigating cotton twice rather than three times a season following UC guidelines will drop yield by about 200 pounds per acre.

With Pima those guides are minus 16 to minus 17 for the first irrigation and minus 19 and 21 for second and subsequent watering.

Giving cotton less than optimal water will also affect the pest populations in cotton, according to a study Munk and University of California regional IPM entomologist/nematologist Pete Goodell. They detailed results of their three years study at the recent West Side Research and Extension Center Field Day Cotton Field Day at Five Points, Calif.

These were not aimed at pest management, but rather giving growers an idea of what might happen if they are unable to give cotton adequate irrigation water.

Impact on pests

Not unexpected, well-water cotton was taller with bigger canopies than deficit irrigated cotton. This has an impact on pests.

Deficit irrigated cotton had fewer lygus than other irrigated treatments on Maxxa in 1991. However, the two scientists said only the so-called luxuriant irrigated regimes had significantly higher populations in 2000 and 2001. Cotton irrigated by UC guidelines did not have excessive lygus.

“There was no difference in lygus between the two and three irrigation treatments” in those two seasons, said Goodell.

The attractiveness of cotton to lygus becomes even more critical late in the summer as crops like processing tomatoes and garlic are harvested. Lush cotton with a heavy canopy could be especially attractive.

Just the opposite was true with aphid populations in two of the three seasons. In 1999 and 2001, aphid populations increased in the deficit-irrigated cotton vs. the other two treatments.

“Drier, stressed cotton should be less attractive than luxuriant cotton, but observations and replicated trials do not support this viewpoint,” said Goodell.